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Internals: Add DfgWorklist commonly used in algorithms (#6361)

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This commit is contained in:
Geza Lore
2025-09-03 13:55:33 +01:00
committed by GitHub
parent 1e4ede08b6
commit 8bf2240d40
3 changed files with 126 additions and 116 deletions
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src/V3DfgPasses.cpp
+42 -87
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@@ -91,23 +91,14 @@ void V3DfgPasses::inlineVars(DfgGraph& dfg) {
}
void V3DfgPasses::removeUnused(DfgGraph& dfg) {
// Map from vertex to next vertex in the work list
DfgUserMap<DfgVertex*> nextp = dfg.makeUserMap<DfgVertex*>();
// Head of work list. Note that we want all next pointers in the list to be non-zero (including
// that of the last element). This allows as to do two important things: detect if an element
// is in the list by checking for a non-zero next pointer, and easy prefetching without
// conditionals. The address of the graph is a good sentinel as it is a valid memory address,
// and we can easily check for the end of the list.
DfgVertex* const sentinelp = reinterpret_cast<DfgVertex*>(&dfg);
DfgVertex* workListp = sentinelp;
// Worklist based algoritm
DfgWorklist workList{dfg};
// Add all unused operation vertices to the work list
for (DfgVertex& vtx : dfg.opVertices()) {
if (vtx.hasSinks()) continue;
// This vertex is unused. Add to work list.
nextp[vtx] = workListp;
workListp = &vtx;
workList.push_front(vtx);
}
// Also add all unused temporaries created during synthesis
@@ -116,45 +107,33 @@ void V3DfgPasses::removeUnused(DfgGraph& dfg) {
if (vtx.hasSinks()) continue;
if (vtx.hasDfgRefs()) continue;
// This vertex is unused. Add to work list.
nextp[vtx] = workListp;
workListp = &vtx;
workList.push_front(vtx);
}
// Process the work list
while (workListp != sentinelp) {
// Pick up the head
DfgVertex* const vtxp = workListp;
// Detach the head
workListp = nextp.at(vtxp);
// Prefetch next item
VL_PREFETCH_RW(workListp);
// This item is now off the work list
nextp.at(vtxp) = nullptr;
workList.foreach([&](DfgVertex& vtx) {
// DfgLogic should have been synthesized or removed
UASSERT_OBJ(!vtxp->is<DfgLogic>(), vtxp, "Should not be DfgLogic");
UASSERT_OBJ(!vtx.is<DfgLogic>(), &vtx, "Should not be DfgLogic");
// If used, then nothing to do, so move on
if (vtxp->hasSinks()) continue;
if (vtx.hasSinks()) return;
// If temporary used in another graph, we need to keep it
if (const DfgVertexVar* const varp = vtxp->cast<DfgVertexVar>()) {
if (const DfgVertexVar* const varp = vtx.cast<DfgVertexVar>()) {
UASSERT_OBJ(varp->tmpForp(), varp, "Non-temporary variable should not be visited");
if (varp->hasDfgRefs()) continue;
if (varp->hasDfgRefs()) return;
}
// Add sources of unused vertex to work list
vtxp->foreachSource([&](DfgVertex& src) {
vtx.foreachSource([&](DfgVertex& src) {
// We only remove actual operation vertices and synthesis temporaries in this loop
if (src.is<DfgConst>()) return false;
const DfgVertexVar* const varp = src.cast<DfgVertexVar>();
if (varp && !varp->tmpForp()) return false;
// If already in work list then nothing to do
if (nextp[src]) return false;
// Actually add to work list.
nextp[src] = workListp;
workListp = &src;
// Add source to workList
workList.push_front(src);
return false;
});
// Remove the unused vertex
vtxp->unlinkDelete(dfg);
}
vtx.unlinkDelete(dfg);
});
// Remove unused and undriven variable vertices
for (DfgVertexVar* const vtxp : dfg.varVertices().unlinkable()) {
@@ -409,32 +388,11 @@ void V3DfgPasses::binToOneHot(DfgGraph& dfg, V3DfgBinToOneHotContext& ctx) {
}
void V3DfgPasses::eliminateVars(DfgGraph& dfg, V3DfgEliminateVarsContext& ctx) {
// Map from vertex to next vertex in the work list
DfgUserMap<DfgVertex*> nextp = dfg.makeUserMap<DfgVertex*>();
// Worklist based algoritm
DfgWorklist workList{dfg};
// Head of work list. Note that we want all next pointers in the list to be non-zero
// (including that of the last element). This allows us to do two important things: detect
// if an element is in the list by checking for a non-zero next pointer, and easy
// prefetching without conditionals. The address of the graph is a good sentinel as it is a
// valid memory address, and we can easily check for the end of the list.
DfgVertex* const sentinelp = reinterpret_cast<DfgVertex*>(&dfg);
DfgVertex* workListp = sentinelp;
// Add all variables to the initial work list
for (DfgVertexVar& vtx : dfg.varVertices()) {
nextp[vtx] = workListp;
workListp = &vtx;
}
const auto addToWorkList = [&](DfgVertex& vtx) {
// If already in work list then nothing to do
DfgVertex*& nextpr = nextp[vtx];
if (nextpr) return false;
// Actually add to work list.
nextpr = workListp;
workListp = &vtx;
return false;
};
// Add all variables to the work list
for (DfgVertexVar& vtx : dfg.varVertices()) workList.push_front(vtx);
// List of variables (AstVar or AstVarScope) we are replacing
std::vector<AstNode*> replacedVariables;
@@ -446,46 +404,40 @@ void V3DfgPasses::eliminateVars(DfgGraph& dfg, V3DfgEliminateVarsContext& ctx) {
bool doReplace = false;
// Process the work list
while (workListp != sentinelp) {
// Pick up the head of the work list
DfgVertex* const vtxp = workListp;
// Detach the head
workListp = nextp.at(vtxp);
// Reset user pointer so it can be added back to the work list later
nextp.at(vtxp) = nullptr;
// Prefetch next item
VL_PREFETCH_RW(workListp);
workList.foreach([&](DfgVertex& vtx) {
// Remove unused non-variable vertices
if (!vtxp->is<DfgVertexVar>() && !vtxp->hasSinks()) {
if (!vtx.is<DfgVertexVar>() && !vtx.hasSinks()) {
// Add sources of removed vertex to work list
vtxp->foreachSource(addToWorkList);
vtx.foreachSource([&](DfgVertex& src) {
workList.push_front(src);
return false;
});
// Remove the unused vertex
vtxp->unlinkDelete(dfg);
continue;
vtx.unlinkDelete(dfg);
return;
}
// We can only eliminate DfgVarPacked vertices at the moment
DfgVarPacked* const varp = vtxp->cast<DfgVarPacked>();
if (!varp) continue;
DfgVarPacked* const varp = vtx.cast<DfgVarPacked>();
if (!varp) return;
if (!varp->tmpForp()) {
// Can't remove regular variable if it has external drivers
if (!varp->isDrivenFullyByDfg()) continue;
if (!varp->isDrivenFullyByDfg()) return;
} else {
// Can't remove partially driven used temporaries
if (!varp->isDrivenFullyByDfg() && varp->hasSinks()) continue;
if (!varp->isDrivenFullyByDfg() && varp->hasSinks()) return;
}
// Can't remove if referenced external to the module/netlist
if (varp->hasExtRefs()) continue;
if (varp->hasExtRefs()) return;
// Can't remove if written in the module
if (varp->hasModWrRefs()) continue;
if (varp->hasModWrRefs()) return;
// Can't remove if referenced in other DFGs of the same module
if (varp->hasDfgRefs()) continue;
if (varp->hasDfgRefs()) return;
// If it has multiple sinks, it can't be eliminated
if (varp->hasMultipleSinks()) continue;
if (varp->hasMultipleSinks()) return;
if (!varp->hasModRefs()) {
// If it is only referenced in this DFG, it can be removed
@@ -504,15 +456,18 @@ void V3DfgPasses::eliminateVars(DfgGraph& dfg, V3DfgEliminateVarsContext& ctx) {
ctx.m_deleteps.push_back(nodep); // Delete variable at the end
nodep->user2p(driverp->nodep());
} else {
// Otherwise this *is* the canonical var
continue;
// Otherwise this *is* the canonical var, keep it
return;
}
// Add sources of redundant variable to the work list
vtxp->foreachSource(addToWorkList);
vtx.foreachSource([&](DfgVertex& src) {
workList.push_front(src);
return false;
});
// Remove the redundant variable
vtxp->unlinkDelete(dfg);
}
vtx.unlinkDelete(dfg);
});
// Job done if no replacements possible
if (!doReplace) return;